What volume of fluid does the brake system hold with a full master cylinder reservoir (or more importantly, how much is needed to flush through and replace old fluid)?

Is there any way of knowing when the "new" fluid has flushed each caliper?

 

You need about 1 litre.
0.5l for flushing and 0.5l for filling the system.

System holds about 0.5 litres.
It doesn't take a lot to drian the pipe work most of the fluid is held in the res.